If you need to intentionally deflate (for air shipment), will the sealant cause a problem? That is, when you push down on your valve core to deflate, will the sealant activate, blocking your valve hole?
Yes, it will treat the valve hole as if it was a puncture. If you can remove the valve core you than will have to clean with a solvent such as white spirit.
Also, the remaining liquid latex will tend to stick the opposing walls of the inner tube together which is why you'd better use a handpump to reinflate and not a brute force approach such as from a CO2 cannister.
I'm not talking about removing the core, just deflating as normal. alerge99 seems to think it's not a huge problem, but it might be bad news if doing multiple deflates/reinflates over the course of a trip, right?
That's not what I'm saying either. I mean, if you can't remove the core you may have a problem in that the latex will block it to the point where you won't be able to reinflate the tubular.
This is especially true if you leave the deflated tubular in that state for a prolonged period of time, long enough so the liquid can harden and obstruct the valve.
Injecting a cleansing product will likely damage the innertube so you're stuck with a dud unless you can build up enough airpressure to blow it through the core into the tyre....
If you can remove the core you can clean it with a spirit and that's quite often the end of that even though I've seen cases where the entire bottom of the stem was blocked off with hardened liquid, piercing it with a blunt object cleared it for a while.
In general deflating the tyre is not recommended once the tyre has been treated either preemptively or for a repair as the inner walls will collapse and tend to bond to each other. This is why I mentioned that care should be taken when reinflating. Go slow here and chances are you'll do just fine.
In the course of a multiple flat within say a day it's no big deal as the liquid won't have enough time to solidify to the full.